It is known in the art (to which the present invention pertains) that useful and desirable circuit functions may be achieved by the utilization of a current or a plurality of currents whose magnitude remains proportional to the absolute temperature of the circuit components. For example, such currents having a magnitude proportional to absolute temperature (PTAT) find application in circuitry designed to provide a constant voltage reference, such as a band-gap reference circuit or, for another example, in circuitry intended to operate as an electronic thermometer for providing a signal representative of temperature. The need for such PTAT current circuits is thus known to exist.
Among the existing technologies, various bipolar technologies for "precision" circuits utilize thin-film resistors in the implementation of proportional to absolute temperature circuits. The reasons for this practice include the fact that such thin-film resistors exhibit a temperature coefficient of resistance that is negligibly small, being considerably smaller than, for example, the temperature coefficient of resistance of an integrated doped resistor as formed in integrated circuit technology. Thus, in the more generally used, less expensive integrated circuit technologies, an integrated resistor exhibits a positive temperature coefficient of resistance typically in the range of 1200 to 1500 parts per million per degree Celsius (ppm/.degree.C.) or greater. When diffusion implanting is utilized with a resistivity in the range of 200 to 2000 ohms per square, a diffused resistor may then exhibit a temperature coefficient of resistance in the order of 3000 to 5000 parts per million per degree Celsius (or per degree Kelvin).
Circuit arrangements are readily provided for deriving a voltage proportional to absolute temperature. Such a voltage may be obtained by taking the difference voltage between the forward-biased base emitter junction voltages (Vbe's) of two bipolar transistors being operated at different emitter current densities. The difference voltage is then applied to the ends of a resistor. If the resistor exhibits a temperature coefficient of resistance that is not too great, then the current in the resistor resulting from the applied voltage difference will exhibit the desired proportionality to absolute temperature.
As is known in the art, the temperature coefficient of the difference voltage between the forward-biased base emitter junction voltages of two bipolar transistors being operated at different emitter current densities in fixed ratio is in the order of 3300 parts per million per degree Celsius. Thus, the temperature coefficient of the voltage is in the order of the temperature coefficient of integrated resistors, as described above. Accordingly, the application to such a resistor of a voltage that is proportional to absolute temperature will, in general, result in a current that is reasonably constant with temperature or which may even exhibit a negative temperature coefficient. While a relatively constant current may be appropriate for the operation of such circuit arrangements for deriving a voltage proportional to absolute temperature, a current proportional to absolute temperature is not thereby obtained.